From reading older posts (back around release), the 4 player Grand Campaign was not recommended for single player due to instability. Is this still the case? Would experienced players recommend using this if you are interested in the option to just control part of an alliance?

Cannot recall any significant stability issues when last I played a four-person scenario using the version 1.08Q patch. That said, I seldom attempt to run more than about two-turns without saving, exiting and re-starting the game. Although the AI might produce odd results as it functions as both your ally and enemy you can tweak it somewhat. Examine the country files found in the ...\World War One Gold\Data\AI\Modding folder ensuring that you save the stock files before editing. Beware for unintended consequences though and good luck.